Tell him how dangerous it is! When i had mine i was told that sex is a no no due to an infection risk, and risk of causing premature labour.
I myself have had a baby at 26 weeks and you want to tell your boyfriend that seeing your child fight for its life is awful.
Your boyfriend sounds like a selfish idiot, my partner had to put up with no sex from me being 6 weeks pregnant, and my daughter is now 16 weeks and we havent been able to resume our sex life, not once has he complained, please be strict with your partner, its not worth the risk